I love the new energy tab in OSX 10.9 mostly because I love data about my machine. I'm curious how the energy impact is calculated, and what the number represents.

It seems clear from using the OS for a week that apps that are running more CUP/memory intense processes (for me: Mathematica and MATLAB) use more energy, or apps that can't take a nap (like listening to pandora in google chrome).

What isn't displayed are some of the bigger energy hogs on a portable: Display settings, WiFi, Bluetooth. I'm wondering if some of this is wrapped into current apps (i.e., if i'm using safari or chrome full screen, will the display power be lumped into this app?) Do display preferences for the app change the energy being used (i.e., working in a text editor with a white-on black scheme instead of black-on white would theoretically use less energy to display).

If these are percents, they do not sum to 100, which is a little annoying.

The energy impact numbers aren't percentages. See this screenshot from an Apple support page; iMovie has an energy impact > 100. In this Tekrevue article they claim a higher number, “[Energy impact] can go from as low as zero to an indefinite high (the highest we’ve seen it is about 780 while running the Geekbench stress test).”
– FlavinNov 4 '13 at 17:00

1 Answer
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I don't think you would want this numbers to be percentile and adding to 100 at any point because that would mean that your battery would die nearly instantaneously.

If I was to make an educated guess just to help you out a bit, your computer (specifically MacBooks) have their battery capacity measure in mAh or milli Ampere-hour typically somewhere in the ballpark of 6500 mAh or 6.5 Ampere-hour. I would hazard a guess (if the moderators allow it) that your are seeing the actual usage to the second of how many mA are going to the specific app per refresh (call time by the Activity Monitor daemon).

So if you were to add up the average power draw of your active apps and multiple that per second you would know how long your computer battery would last without power (roughly). Additionally you have to factor in what internal components are working hard (AirPort, Bluetooth, Display brightness, hard drive rotations) and you have yourself the estimated time until depletion.

Oh, and as to how they are calculated. There is a tiny integrated circuit on your main logic board called the SMC and it's job is to allocate DC current to the resource that needs it most. It keeps track of energy usage and demands by the RAM, CPU, GPU, etc. so that it can help make sure your computer stays energy efficient.